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authorGravatar Eric Andersen <andersen@codepoet.org>2001-11-22 00:15:59 +0000
committerGravatar Eric Andersen <andersen@codepoet.org>2001-11-22 00:15:59 +0000
commitc117dd5fb183afb1a4790a6f6110d88704be6bf8 (patch)
tree13ce27b066f424accc986a0f2c915c144d332861
parentee185898f61c50ad0b174b45a58c38054571a0b3 (diff)
downloaduClibc-c117dd5fb183afb1a4790a6f6110d88704be6bf8.tar.gz
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Seems we were lacking an acos() implementation
-rw-r--r--libm/double/acos.c58
1 files changed, 58 insertions, 0 deletions
diff --git a/libm/double/acos.c b/libm/double/acos.c
new file mode 100644
index 000000000..60f61dc98
--- /dev/null
+++ b/libm/double/acos.c
@@ -0,0 +1,58 @@
+/* acos()
+ *
+ * Inverse circular cosine
+ *
+ *
+ *
+ * SYNOPSIS:
+ *
+ * double x, y, acos();
+ *
+ * y = acos( x );
+ *
+ *
+ *
+ * DESCRIPTION:
+ *
+ * Returns radian angle between 0 and pi whose cosine
+ * is x.
+ *
+ * Analytically, acos(x) = pi/2 - asin(x). However if |x| is
+ * near 1, there is cancellation error in subtracting asin(x)
+ * from pi/2. Hence if x < -0.5,
+ *
+ * acos(x) = pi - 2.0 * asin( sqrt((1+x)/2) );
+ *
+ * or if x > +0.5,
+ *
+ * acos(x) = 2.0 * asin( sqrt((1-x)/2) ).
+ *
+ *
+ * ACCURACY:
+ *
+ * Relative error:
+ * arithmetic domain # trials peak rms
+ * DEC -1, 1 50000 3.3e-17 8.2e-18
+ * IEEE -1, 1 10^6 2.2e-16 6.5e-17
+ *
+ *
+ * ERROR MESSAGES:
+ *
+ * message condition value returned
+ * asin domain |x| > 1 NAN
+ */
+
+#define __USE_BSD
+#include <math.h>
+
+double acos(double x)
+{
+ if (x < -0.5) {
+ return (M_PI - 2.0 * asin( sqrt((1+x)/2) ));
+ }
+ if (x > 0.5) {
+ return (2.0 * asin( sqrt((1-x)/2) ));
+ }
+
+ return(M_PI_2 - asin(x));
+}